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Ensembles and Probabilistic Forecasting


... prediction at NCEP and ECMWF (European Centre for Medium Range Weather Forecasts) ... United Kingdom Meteorological Office Diff. ~ 60 km 'Native' Models ... – PowerPoint PPT presentation

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Title: Ensembles and Probabilistic Forecasting

Ensembles and Probabilistic Forecasting
Probabilistic Prediction
  • Because of forecast uncertainties, predictions
    must be provided in a probabilistic framework,
    not the deterministic single answer approach that
    has dominated weather prediction during the last
  • Interestinglythe first public forecasts were

Ol Probs
Cleveland Abbe (Ol Probabilities), who led the
establishment of a weather forecasting division
within the U.S. Army Signal Corps. Produced the
first known communication of a weather
probability to users and the public in 1869.
Professor Cleveland Abbe, who issued the first
public Weather Synopsis and Probabilities on
February 19, 1871
The Trend to Deterministic Forecasts During the
Later 19th and First Half of the 20th Centuries.
Foundation for probabilistic prediction
  • The work of Lorenz (1963, 1965, 1968)
    demonstrated that the atmosphere is a chaotic
    system, in which small differences in the
    initializationwell within observational error
    can have large impacts on the forecasts,
    particularly for longer forecasts.
  • Not unlike a pinball game.

  • Similarly, uncertainty in our model physics also
    produces uncertainty in the forecasts.
  • Lorenz is a series of experiments demonstrated
    how small errors in initial conditions can grow
    so that all deterministic forecast skill is lost
    at about two weeks.
  • Talked about the butterfly effect

  • The Lorenz Diagramchaos
  • Is not necessarily random

Probabilistic NWP
  • To deal with forecast uncertainty, Epstein (1969)
    suggested stochastic-dynamic forecasting, in
    which forecast errors are explicitly considered
    during model integration, but this method was not
    computationally practical.
  • Another approach, ensemble prediction, was
    proposed by Leith (1974), who suggested that
    prediction centers run a collection (ensemble) of
    forecasts, each starting from a different initial
    state. The variations in the resulting forecasts
    could be used to estimate the uncertainty of the
    prediction. But even the ensemble approach was
    not tractable at this time due to limited
    computer resources.

Ensemble Prediction
  • Can use ensembles to provide a new generation of
    products that give the probabilities that some
    weather feature will occur.
  • Can also predict forecast skill!
  • It appears that when forecasts are similar,
    forecast skill is higher.
  • When forecasts differ greatly, forecast skill is
  • To create a collection of ensembles one can used
    slightly different initializations or different

Ensemble Prediction
  • By the early 1990s, faster computers allowed the
    initiation of global ensemble prediction at NCEP
    and ECMWF (European Centre for Medium Range
    Weather Forecasts).
  • During the past decade the size and
    sophistication of the NCEP and ECMWF ensemble
    systems have grown considerably, with the
    medium-range, global ensemble system becoming an
    integral tool for many forecasters. Also during
    this period, NCEP has constructed a higher
    resolution, short-range ensemble system (SREF)
    that uses breeding to create initial condition

NCEP Global Ensemble System
  • Begun in 1993 with the MRF (now GFS)
  • First tried lagged ensembles as basisusing
    runs of various initializations verifying at the
    same time.
  • For the last ten years have used the breeding
    method to find perturbations to the initial
    conditions of each ensemble members.
  • Breeding adds random perturbations ( and -) to
    an initial state, let them grow, then reduce
    amplitude down to a small level, lets them grow
    again, etc.
  • Give an idea of what type of perturbations are
    growing rapidly in the period BEFORE the
  • Does not include physics uncertainty.
  • Coarse spatial resolution..only for synoptic

NCEP Global Ensemble
  • At 00Z
  • T254L64 high resolution control out to 7 days,
    after which this run gets truncated--just larger
    scales and is run out to 16 days at a T170L42
  • T62 control that is started with a truncated T170
  • 10 perturbed forecasts each run at T62 horizontal
    resolution. The perturbations are from five
    independent breeding cycles.
  • At 12Z
  • T254L64 control out to 3 days that gets truncated
    and run at T170L42 resolution out to 16 days
  • Two pairs of perturbed forecasts based on two
    independent breeding cycles (four perturbed
    integrations out to 16 days).

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U.S. Navy Also Has A Global Ensemble System Using
NCEP Short-Range Ensembles (SREF)
  • Resolution of 32 km
  • Out to 87 h twice a day (09 and 21 UTC
  • Uses both initial condition uncertainty
    (breeding) and physics uncertainty.
  • Uses the Eta and Regional Spectral Models and
    recently the WRF model (21 total members)

SREF Current System
Model Res (km) Levels Members Cloud
Physics Convection RSM-SAS 45 28 Ctl,n,p
GFS physics Simple Arak-Schubert RSM-RAS
45 28 n,p GFS physics Relaxed
Arak-Schubert Eta-BMJ 32 60 Ctl,n,p Op
Ferrier Betts-Miller-Janjic Eta-SAT
32 60 n,p Op Ferrier BMJ-moist
prof Eta-KF 32 60 Ctl,n,p Op
Ferrier Kain-Fritsch Eta-KFD 32 60 n,p Op
Ferrier Kain-Fritsch with
enhanced detrainment
PLUS NMM-WRF control and 1 pert. Pair
ARW-WRF control and 1 pert. pair
UW Short Range Ensemble System
UW Mesoscale Ensemble System
  • Single limited-area mesoscale modeling system
  • 2-day (48-hr) forecasts at 0000 UTC and 12 UTC in
    real-time since January 2000. 36 and 12-km

UW Ensemble System
  • UW system is based on the use of analyses and
    forecasts of major operational modeling centers.
  • The idea is that differences in initial
    conditions of various operational centers is a
    measure of IC uncertainty.
  • These IC differences reflect different data
    inventories, assimilation schemes, and model
    physics/numerics and can be quite large, often
    much greater than observation errors.
  • In this approach each ensemble member uses
    different boundary conditions--thus finessing the
    problem of the BC restraining ensemble spread.
  • Also include physics diversity

Native Models/Analyses Available
Resolution ( _at_ 45 ?N )
Objective Abbreviation/Model/Source
Type Computational Distributed Analysis
avn, Global Forecast System (GFS),
Spectral T254 / L64 1.0? / L14 SSI National
Centers for Environmental Prediction 55 km 80
km 3D Var   cmcg, Global Environmental
Multi-scale (GEM), Finite 0.9??0.9?/L28 1.25? /
L11 3D Var Canadian Meteorological Centre Diff
70 km 100 km   eta, limited-area mesoscale
model, Finite 32 km / L45 90 km /
L37 SSI National Centers for Environmental
Prediction Diff. 3D Var   gasp, Global
AnalysiS and Prediction model, Spectral T239 /
L29 1.0? / L11 3D Var Australian Bureau of
Meteorology 60 km 80 km jma, Global Spectral
Model (GSM), Spectral T106 / L21 1.25? /
L13 OI Japan Meteorological Agency 135 km 100
km   ngps, Navy Operational Global Atmos. Pred.
System, Spectral T239 / L30 1.0? / L14 OI Fleet
Numerical Meteorological Oceanographic Cntr.
60 km 80 km tcwb, Global Forecast
System, Spectral T79 / L18 1.0? / L11 OI Taiwan
Central Weather Bureau 180 km 80 km   ukmo,
Unified Model, Finite 5/6??5/9?/L30 same /
L12 3D Var United Kingdom Meteorological Office
Diff. 60 km
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Relating Forecast Skill and Model Spread
Mean Absolute Error of Wind Direction is Far Less
When Spread is EXTREME (Low or High)
Ensemble-Based Probabilistic Products
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The Thanksgiving Forecast 2001 42h forecast
(valid Thu 10AM)
SLP and winds
  • Reveals high uncertainty in storm track and
  • Indicates low probability of Puget Sound wind

1 cent
11 ngps
5 ngps
8 eta
2 eta
3 ukmo
12 cmcg
9 ukmo
6 cmcg
4 tcwb
13 avn
10 tcwb
7 avn
Ensemble-Based Probabilistic Products
Ensemble Prediction in the U.S.
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